The overall goal of this proposal is to gain a better understanding of the function of Ras proteins. They play an important role in the transduction of signals emanating from a variety of cell surface receptors. Furthermore, when Ras proteins are constituitively activated they contribute to the formation of a wide variety of human tumors. Ras proteins belong to a large superfamily of GTP binding proteins that cycle between the active GTP-bound and inactive GDP-bound forms. Activation occurs by the release of GDP and its replacement with GTP. This event is catalyzed by Guanine-nucleotide Releasing Factors (GRFs). Inactivation occurs by hydrolysis of GTP to GDP. This process is catalyzed by GTPase Activating Proteins (GAPs). The proposed experiments focus on Guanine-nucleotide Releasing Factors that activate Ras proteins (Ras-GRFs). We plan to elucidate the biochemical mechanisms that regulate Ras-GRF activity. In this way we hope to reveal at least some of the biochemical events that lead to Ras activation in response to ligand-bound cell surface receptors. We will also investigate whether elevated levels of Ras-GRF activity in cells lead to constituitively active Ras and oncogenic transformation. This would suggest that a Ras-GRF constitutively activated by mutation could contribute to human oncogenesis. Moreover, mutagenic studies will be used to define amino acids that participate in the interaction between Ras and its GRFs. Because the brain-specific Ras-GRF we have recently cloned has a separate domain that may activate a different GTP binding protein family, this investigation may also reveal how multiple signals triggered by receptors can be coordinately regulated. Finally, we plan to study the regulation of additional members of what appears to be a family of Ras-GRFs, since this may contribute to our understanding of tissue-specific Ras function.